1. Technical Field
This invention relates generally to the art of powder metallurgy, and more particularly to iron-based products such as valve guides and valve seat inserts.
2. Related Art
Powder metal valve guides, valve seat inserts and other high temperature wear components are often manufactured from iron-based powder mixtures or pre-alloy powders. In the case of mixtures, various powder additives are combined with elemental iron powder to provide lubricity, wear resistance, machineability and high temperature strength. One common additive employed as a solid lubricant is molybdenum disulfide (MOS2). MoS2 has a double layered structure of sulfur held together by weak Van-der-Waals forces which shear under pressure to provide good lubrication at levels of about 2-3 wt. % of the mixture. While MoS2 is an excellent solid lubricant, it has a tendency to expand during sintering when present in amounts normally used for solid lubricant, particularly when present in a mixture containing carbon and copper. The high distortion associated with MoS2 (as much as 1% or more change in dimension from compaction to sintering) is detrimental to the manufacture of low cost, high precision net shape articles such as valve guides and valve seat inserts, and thus MoS2 as a lubricant is typically avoided in such powder metal applications.
U.S. Pat. No. 5,507,257 discloses an iron-based powder metal mixture for valve guide applications which contains additions of coarse and fine graphite powder together with the addition of ferrophosphorus or cuprophosphorus powders. The resultant sintered articles contain hard Fexe2x80x94Cxe2x80x94P dispersions in the iron matrix together with a certain amount of free graphite from the coarse graphite powder. The patent further reports the formation of carbides when the mixture contains molybdenum powder. Phosphorus is known in the art to have a stabilizing effect on the xcex1-iron. Low carbon solubility in the xcex1-iron phase promotes the presence of free graphite in the sintered article which is beneficial as a solid lubricant. In addition, phosphorus is known to accelerate sintering through formation of a transient liquid phase. While phosphorus stabilizes the xcex1-iron phase and promotes sinterability, it is also detrimental in that the partial liquid phase sintering causes shrinkage upon solidification to such a degree that the tolerances of sintered products for net-shape applications may be adversely affected At high carbon contents greater than 0.2 wt. %, hard phosphorus compounds and cementite form at the grain boundaries as a result of the partial liquid phase sintering and are detrimental to Vie machineability of the parts. For at least these reasons, the addition of phosphorus in iron-based net-shape powder metal applications is generally undesirable for its detrimental effect on net-shape stabilization and machineability.
Iron-based sintered powder metal articles according to the invention are fabricated from an iron-based powder metal mixture consisting essentially of, by weight: 0.5-2.5% stable graphite having a mesh size of about 325 to 100, 0.5-2.5% soluble graphite having a mesh size greater than 325, 0.5-3.0% MoS2, 1.0-5.0% Cu, and the balance to Fe and impurities.
MoS2, when combined with carbon and copper in an iron-based system has shown to react favorably with carbon and copper to promote sinterability even at low temperatures of between 1030-1150xc2x0 C. while achieving good levels of material strength which are normally attained at higher sintering temperatures. The reaction during sintering is advantageously a solid state reaction, avoiding the formation of a transient liquid phase which occurs for example, with the addition of phosphorus, known to be detrimental to dimensional stability of net-shaped articles such as valve guides and valve seat inserts. The relatively low MoS2 additions work in sinergy with the additions of stable and soluble graphite to achieve the desired properties of good strength, wear resistance and machineability. The additions of the relatively fine soluble graphite reacts sacrificially with the elemental iron powder during sintering in a solid state reaction to retain coarse graphite. The relatively coarse stable graphite is, effectively, insoluble in the iron and it is present in the sintered article as free graphite to promote good machineability of the sintered article. The combined carbon promotes good strength and wear resistance.
A significant cost saving is realized by sintering the articles at 20 relatively low sintering temperatures of 1030-1150xc2x0 C., and particularly at about 1050xc2x0 C. Added strength and wear resistance can be attained when sintering the articles at the higher end of the sintering range (i.e., toward 1150xc2x0 C.), wherein the MoS2 reacts to form molybdenum carbides, while the stable graphite retains the presence of free graphite in the sintered structure for achieving good machineability. Thus, the powder metal mixture according to the invention enables articles to be compacted and sintered at relatively low sintering temperatures as compared to conventional sintering temperatures for iron-based powders having similar properties, while achieving high strength and wear resistance with or without the formation of molybdenum carbides while attaining excellent machineability and dimensional stability of the sintered structure.
The invention also contemplates a method of forming sintered articles, wherein an iron-based powder mixture is prepared according to the above composition and then is compacted and sintered at temperatures between 1030-1150xc2x0 C. to achieve a high strength, high wear resistant article having the characteristics of good machineability and excellent dimensional stability for net-shape, high temperature wear applications.